Most essential oils consist of mixtures of hydrocarbons (terpenes, sesquiterpenes, etc.), oxygenated compounds (alcohols, esters, ethers, aldehydes, ketones, lactones, phenols, phenol ethers, etc.), and a small percentage of viscid or solid nonvolatile residues (paraffins, waxes, etc.). Of these the oxygenated compounds are the principal odor carriers, although the terpenes and sesquiterpenes, too, contribute in some degree to the total odor and flavor value of the oil. The oxygenated substances possess the added advantage of better solubility in dilute alcohol and, with the exception of some aldehydes, of greater stability against oxidizing and resinifying influences. Due to their unsaturated character, the terpenes and sesquiterpenes oxidize and resinify easily under the influence of air and light or under improper storing conditions which means spoilage of odor and flavor, and lowering of the solubility in alcohol.
For many years, therefore, it has been the endeavor of the essential oil industry to supply the users with concentrated, terpeneless and sesquiterpeneless oils. Such oils consist mainly of oxygenated compounds; they are more soluble, more stable, and much stronger in odor, yet retain most of the odor and flavor characteristics of the original oil.
The degree of concentration is automatically limited by the amount of oxygenated compounds present in the natural oil. For example, an orange oil containing only 2 per cent of oxygenated constituents and 98 per cent of terpenes, sesquiterpenes and waxes can, theoretically, be concentrated fifty times at the most, whereas a bergamot oil containing 50 per cent esters, alcohols, lactones, etc., and 50 per cent hydrocarbons can be concentrated only to double strength.
Before discussing in more detail the methods of manufacturing these concent rated, torpcnclcss and sesquiterpenoless oils, we should point out for clarity's sake that they must not be confused with the so-called isolates or aromatic isolates, or commonly but incorrectly called "synthetics" which are isolated from certain essential oils. For instance, citral can be isolated by fractionation or by chemical means from lemongrass oil, eugenol from clove oil, safrol from sassafras oil or camphor oil fractions, citronellal from citronella oil. These isolates may be converted chemically into other compounds, real synthetics, viz., citral into ionones, eugenol into vanillin, safrol into holiotropin, citronellal into citronellol, citronellyl acetate, hydroxycitronellal or synthetic menthol. Terpeneless and sosquiterpojieloss oils have nothing to do with these isolates as the latter consist usually of only one well defined chemical substance, while the former are composed of several, often many, oxygenated compounds as present in the normal essential oil.
Because of the different composition, the deterpcnation of each essential oil requires a special process. The general method is based upon two principles: (a) removal of the terpenes, sesquiterpenes and paraffins by fractional distillation in vacuo or (b) by extraction of the more soluble oxygenated compounds with dilute alcohol or other solvents. In many cases, especially with citrus oils, a combination of the two methods may be employed.
The commercial term "sesquiterpeneless" oils conventionally includesalso the terpeneless oils. In some cases, especially when the content of sesquiterpenes in the natural oil is small, the two terms are employed synonymously. The trade designations and the names of the many brands on the market, however, are not always correct from the scientific point of view.
It would be more appropriate to name these products "Concentrated Oils," "Terpeneless Oils," and "Terpeneless and Sesquiterpeneless Oils."
"Concentrated oils" are those from which only a part of the hydrocarbons have been removed. This can be done by simple fractional distillation in vacuo. According to the process applied and the intended concentration, a wide range of concentrated oils, with different properties, may be obtained. Thus, we speak of a twofold lemon or orange oil, a fivefold oil, etc. "Terpeneless oils" are those from which all or most of the terpenes and waxes have been removed, usually by fractional distillation. "Terpeneless and Sesquiterpeneless oils" are those from which the terpenes, the sesquiterpenes and the waxes have been eliminated. The common manufacturing practice is to distill off in vacuo first the terpenes, and then to extract the terpeneless oil with dilute alcohol, or other solvents, whereby the sesquiterpenes and waxes are eliminated ; or, the sesquiterpenes and waxes may be removed by further fractionation of the terpeneless oil in vacuo. The resulting terpeneless and Sesquiterpeneless oil represents the highest possible concentration of a natural essential oil.
The manufacture of these products requires that the operator be well acquainted with the chemical composition, especially with the boiling ranges of the various terpenes, sesquiterpenes and oxygenated compounds occurring in the natural oil which he expects to concentrate. The boiling range of terpenes varies in most cases from 150o to 180o at atmospheric pressure; that of sesquiterpenes from 240o to 280o. The boiling points of most oxygenated compounds (terpene alcohols, aldehydes, esters, etc.; lie between those of the terpenes and sesquiterpenes. Phenols, phenol ethers, and a few aromatic aldehydes form an exception, also the sesquiterpene alcohols, esters, etc., their boiling range falling into that of the sesquiterpenes or above.
As far as solubility in dilute alcohol is concerned, the terpenes are, in general, only sparingly soluble, the paraffins and sesquiterpenes practically insoluble. The oxygenated compounds, on the other hand, possess in general much better solubility : the alcohols, aldehydes, ketones, and phenols are most soluble, the esters and phenol ethers somewhat less soluble.
As pointed out, the terpenes may be removed by fractional distillation of the natural oil under reduced pressure. Most constituents of essential oils being deleteriously affected by heat, the distillation temperature must be kept as low as possible, which can be achieved with the aid of a good vacuum. For best results a well-constructed fractionation still as described in the section on "Distillation of Essential Oils" should be employed. It must be equipped with an efficient fractionation column.
It should be borne in mind that the terpenes cannot be removed quantitatively from a natural oil by mere fractional distillation; indeed, one of the greatest disadvantages of fractional distillation lies in the incomplete separation of the constituents, especially if their boiling points lie close together. A typical example is lemon oil which, aside from citral, contains also lower boiling aldehydes, such as octyl, nonyl and decyl aldehyde. If natural lemon oil is fractionated at 2 mm. pressure, the lower boiling terpenes should come over first, and theoretically the terpene fraction should contain no citral. However, even with a very efficient fractionation column, the aldehyde content of the terpene fraction will amount to about 1.0 per cent. The terpene fraction may be refractionated, but it will still retain small quantities of aldehydes; furthermore, repeated heating affects the flavor. Separation of the oxygenated compounds by chemical means is limited to certain cases only.
Repeated fractionation results in several intermediary fractions which consist of terpenes and a slight amount of oxygenated compounds, the latter increasing in proportion as the distillation temperature rises. Fractionation may be conducted in such a way that the residual oil is free from terpenes, but in this case the residual oil will be deprived also of those portions of the oxygenated constituents which have been carried over into the intermediary fractions. In order to recover these compounds, it will be necessary to refractionate the intermediary fractions, but, as said, prolonged heating is likely to have a deleterious effect upon the odor and especially the flavor of the fractions. Fractionation may be controlled by testing each fraction for solubility and for its rotatory power.
The elimination of the sesquiterpenes presents even more difficulties than that of the terpenes. In some cases the sesquiterpenes may be separated from the terpeneless oils by mere fractionation in vacuo, provided that the oils are not affected by the relatively high boiling temperature required for the distillation of scsquiterpenes (about 120o-140o at 10 mm. pressure) and by the partial overheating in the still which easily takes place. A vacuum of 3 to 5 mm. is desirable. In this case, too, the manufacturer must be familiar with the boiling points, at reduced pressure, of the various oil constituents. In some cases the differential in the boiling points of two compounds, as prevailing at atmospheric pressure, does not remain constant at reduced pressures; it may even be reversed. Too, every fraction should be tested for its rotatory power and for solubility in dilute alcohol, the insoluble ones to be rejected as containing mainly sesquiterpenes. Refractionation of the rejected fractions may be necessary. Even at a pressure of only 1 mm., a relatively high temperature is required to distill over the oxygenated compounds, most of them boiling between 90o-110o. Moreover, the temperature in the still itself will usually be about 10o and even 20o higher than the boiling point of the liquid, and intense local heating occurs especially along the walls of the still. All these factors tend to impart to the oil a note which the expert easily recognizes as "distilled" or slightly "burnt," as it does not occur in natural cold-pressed citrus oils, for example. Furthermore, the influence of heat seems to decompose the so-called "molecular compounds" which some authorities assume to occur in natural oils. It is a well-known fact that, upon aging, the odor of a perfume or flavor mixture changes and improves considerably. This may be caused by chemical reactions of functional groups for example, by the interaction of alcohols and aldehydes which form acetals. Such compounds may exist in the natural oil and be decomposed upon heating and distilling.
Another method of removing the high boiling sesquiterpenes and waxes from the terpeneless oil consists in steam distilling the terpeneless oil at reduced pressure. This process is more gentle than dry distillation in vacua and leaves the high boiling sesquiterpenes anc! waxes as residues in the still. In this case the distillate should be tested for solubility; any sesquiterpenes distilled over may be removed by treating the fractions with dilute alcohol. This method, however, has the inherent disadvantage that compared with dry vacuum distillation it takes much longer, especially in the case of oils containing a large percentage of high boiling compounds. Also, certain constituents of an oil are liable to dissolve in the distillation water e.g., phenylethyl alcohol or eugenol. In this case the distillation water has to be returned into the still for cohobation.
In view of these inadequacies, some manufacturers remove all remaining terpenes and sesquiterpenes from concentrated oil by extracting the latter with dilute alcohol. The strength of the alcohol to be employed for this purpose depends primarily upon the solubility of the oxygenated compounds. Thus, the concentrated oil from which most of the torpenes and sesquiterpenes have been eliminated by fractionation in vacua or by steam distillation under reduced pressure is shaken for some time with fifteen to twenty times its volume of dilute alcohol, for instance, with 60 per cent alcohol by volume ; or the concentrated oil is first dissolved in the corresponding volume of strong alcohol and then the required amount of distilled water is gradually added with continuous stirring until the desired degree of alcohol dilution is reached. In both cases the turbid mixture should be cooled for a prolonged period and set aside until clarified. Thus, the oxygenated constituents dissolve in the dilute alcohol, while the terpenes and sesquiterpenes remain undissolved and (together with traces of oxygenated compounds) may be separated.
Because of the small differential in the specific gravity of the undissolved parts of the oil and that of the solution, emulsions may form and the separation of the two layers may require some time. In order to break the emulsion, small quantities of low boiling petrol ether are added, or the emulsion may be separated by centrifuging. The undissolved oil is repeatedly treated with dilute alcohol in order to extract any quantities of oxygenated compounds which it might still retain.
The clear solution of oxygenated compounds in dilute alcohol is then transferred into a still and the alcohol fractionated off at reduced pressure, until only oil and water remain in the still. The two layers of oil and water can easily be separated. The employment of an efficient condenser will prevent losses of alcohol. The recovered alcohol and the residual water may be used again for treating the next batch of oil.
The literature on essential oils contains many references to the preparing of terpeneless and sesquiterpeneless oils, one of the most comprehensive ones being the paper by Littlejohn.46 As stated, no standard method has been adopted yet and every manufacturer uses his own process. Romeo47 reported that terpeneless and sesquitcrpeneless citrus oils are manufactured in Sicily by first removing the terpenes by fractional distillation. The sesquiterpenes are then eliminated from the terpeneless oil by extracting the oil with dilute alcohol, the strength of which should be somewhat lower than that in which the sesquiterpeneless oils must finally be soluble. The sesquiterpeneless oil is separated from the alcoholic solution by the addition of water or by distilling off the alcohol under reduced pressure. This constitutes the general method described previously and with some modifications it forms, today, the basis of most commercial processes.
A more novel method has been described and patented by van Dijek and Ruys.48 In this process the natural oil is extracted by two solvents which are only partially soluble in one another for instance, pentane and dilute methyl alcohol. The two solvents are made to flow, according to the countercurrent principle, through a horizontal glass cylinder and the oil is entered in the middle. The terpenes dissolve in the pentane phase, the oxygenated compounds in the methylalcohol phase. After separation of the two phases, the solvents are removed by distillation, only low temperatures being necessary. This, according to the inventors, is the principal advantage of thoir method, aside from the fact that high-grade terpeneless oils are obtained in almost quantitative yield. The principal difficulties of this process lie in the necessity of working with large volumes of solvents, furthermore, in the tendency toward formation of emulsions which, however, might be broken in some cases by the addition of 0.1 per cent of citric or tartaric acid.
After having discussed the various methods of manufacturing terpeneless and sesquiterpenoless oils, it might be advisable to add a few words about their concentration, as there exists a great deal of confusion regarding this point. The price lists and tables on the concentration of terpeneless and sesquiterpeneless oils issued by the various essential oil houses differ widely in regard to their concentration value. Yet, the theoretical concentration could be calculated only from the actual yield of terpeneless or sesquiterpeneless oil as obtained from a given weight of natural oil.
46 Flavours 3, No. 4, August (1940), 7.
47 La deterpenazione delk essenze di agrumi. Estr. dagli Atti del H. Congresso Nazionak di Chimica pura ed applicata. Palermo, May (1926). Ber. Schimmel & Co. (1928), 38.
48 Perfumery Essential Oil Record 28 (1937), 91.

However, the actual odor and flavor strength of two oils, although of the same theoretical concentration, may differ, concentration not being necessarily proportionate to odor and flavor strength. Let us assume, for instance, that 100 kg. of natural lemon oil are converted into terpeneless oil and that the yield is 8 kg. of terpeneless oil containing about 40 to 45 per cent of citral. (Some citral has been destroyed by distillation and, besides, the oxygenated compounds cannot be completely freed of terpenes.) In this case the actual concentration of the oil, but not necessarily of the flavor, is obviously twelve and one-half times.
It is difficult, if not impossible, to indicate general and definite limits for the physicochemical properties of concentrated, terpeneless and sesquiterpeneless oils because of the fact that these properties depend upon the degree of concentration and upon the relative proportions of oxygenated constituents originally present. Furthermore, every manufacturer has his own standards which are based upon his particular manufacturing process. Littlejohn49 listed the physicochemical properties of more than fifty terpeneless and sesquiterpeneless oils. Accordinging to this author, the specific gravity affords a valuable clue to the presence of any remaining terpenes. These hydrocarbons possess a low specific gravity and refractive index and their complete removal should raise the specific gravity and refractive index of the terpeneless oil relative to that of the original oil. The determination of the optical rotation, too, provides a good indication regarding the extent to which the terpenes have been eliminated.
By far the most important criterion for a terpeneless and sesquiterpeneless oil is its solubility in dilute alcohol, 70 per cent ethyl alcohol usually being employed for this purpose. A terpeneless oil should usually be soluble in 3 to 10 volumes of 70 per cent alcohol, while a sesquiterpeneless oil should be more soluble.
Aside from the determination of the physicochemical properties, it is advisable to test a terpeneless or sesquiterpeneless oil also for its content of oxygenated compounds, especially for alcohols, esters and aldehydes, which can be done by the usual analytical methods. Bocker60 suggested a method of evaluating and examining terpeneless lemon oils which is based on treating the aldehyde-free oil with 51 per cent alcohol to remove all oxygenated compounds, and on measuring the quantity of terpenes and sesquiterpenes left. For this purpose, 10 cc. of the oil is first treated with a solution of neutral sodium sulfite which removes the aldehydes. The remaining, unabsorbed oil is shaken with 100 times its volume of 51 per cent alcohol in a separatory funnel, cooled to about 2 and left for a period of 6 hr. or more, until the liquids have completely separated when the lower layer can be removed.
49 Flavours 3, No. 4, August (1940), 7.
50 J. prakt. Chem. [2], 89 (1914), 199; [2], 90 (1914), 393.

After washing the oil layer with a further quantity of 51 per cent alcohol, all undissolved oil is transferred to a burette tube and its volume carefully measured. From this amount the percentage of nonoxygenated constituents of the original oil can be determined. In order to obtain more exact results the terpeneless oil is first fractionated and the process applied to both the first and the last fraction. Docker's method is not absolutely quantitative, as some terpenes will dissolve in the weak spirit, also because the transfer of the oils from the separatory funnel to the measuring burette always causes some loss.
The main advantage of the terpeneless and^ especially of the sesquiterppnoless oils consists in their better solubility in dilute alcohol. The employment of these oils, therefore, ejects a considerable saving of alcohol in the finished goods; odor and flavor of the oil are better utilized. A further advantage consists in the fact that, by the process of concentration, the oils are also freed of any products~bf decomposition or resinification which might result from improper handling or aging of the natural oils. Another merit of the terpeneless and sesquiterpeneless oils lies in their better stability. While natural citrus oils are apt to resinify, primarily due to polymerization of certain hydrocarbons, the concentrated oils are much more stable. Thus, they may be employed in powders, for the flavoring of gelatin desserts, for example, or for the scenting of bath salts.
The introduction of terpeneless and sesquiterpeneless oils on the market has met with some resistance. Several authorities contend that the elimination of terpcnes and sesquiterpenes removes also a part of the characteristic odor and flavor of the natural oil. The application of heat undoubtedly has some effect on the delicate flavoring constituents of the oil and, if improperly prepared, concentrated oils may not display the freshness and bouquet of the original oil. Furthermore, the terpeneless and sesquiterpeneless oils contain a lower proportion of natural fixatives such as waxes and stearoptenes which contribute to the retaining of the flavor on the palate. Weighing the advantages against the disadvantages, the conclusion may be drawn that concentrated, terpeneless and sesquiterpeneless oils have their definite place in many formulas where highest possible concentration, solubility, and stability are required, but that they cannot replace the natural oils for all purposes.
"Concentrated Citrus Oils," by A. H. Bennett, Perfumery Essential Oil Record 25 (1934), 111.
"Preparation of Terpeneless Oils," by A. M. Burger, Riechstoff Ind. 13 (1938), 217. Chem. Abstracts 33 (1939), 1089.
"Removal of Terpenes from Essential Oils," by Pietro Leone, Riv. ital, essenze profumi 28 (1946), 5, 39, 82. Chem. Abstracts 40 (1946), 5201.
"Terpeneless and Sesquiterpeneless Oils," by Y. R. Naves, Mfg. Chemist 18, No. 4 (1947), 173.

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